The invention relates to structured polyacrylates crosslinked by selective UV irradiation, to their use, and to a process for preparing them.
In the field of pressure-sensitive adhesive (PSA) compositions, ongoing technological developments in the coating technique mean that there is a progressive need for new developments. Within the industry, hotmelt processes with solventless coating technology are of increasing importance in the preparation of PSA compositions, since the environmental regulations are becoming ever greater and the prices of solvents continue to rise. The consequence of this is that solvents are to be eliminated as far as possible from the manufacturing process for PSA tapes. The associated introduction of the hotmelt technology is imposing ever greater requirements on the adhesive compositions. Acrylic PSA compositions in particular are the subject of very intensive investigations aimed at improvements. For high-level industrial applications, polyacrylates are preferred on account of their transparency and weathering stability. In addition to these advantages, however, these acrylic PSA compositions must also meet stringent requirements in respect of shear strength and bond strength. This profile of requirements is met by polyacrylates of high molecular weight and high polarity with subsequent efficient crosslinking. These high shear strength, polar PSA compositions possess the disadvantage, however, that they are unsuited to the hotmelt extrusion process, because high application temperatures are necessary and because, furthermore, the molecular weight of the polymer is reduced by shearing in the extruder. This damage significantly lowers the level of the adhesive properties. The bond strength and the tack are generally low, since owing to the polar fractions in the adhesive compositions the glass transition temperature is relatively high. The shear strengths of the hotmelt-coated acrylic PSA compositions, in particular, fall distinctly in comparison to the original, solvent-coated PSA composition. At the present time, therefore, different concepts aimed at reducing the flow viscosity and thereby facilitating extrusion coating of these PSA compositions are being investigated.
The industry is pursuing a variety of concepts for achieving this objective. One possibility is the highly efficient crosslinking of a low-viscosity, nonpolar acrylic adhesive composition only when it is on the backing. Acrylates containing electron donating groups are copolymerized and, during crosslinking by UV or EBC (EBC: electron beam curing), they stabilize free radicals that are formed. Examples are tertiary amine monomers [WO96/35725], tertiary butylacrylamide monomer [U.S. Pat. No. 5,194,455], and the tetrahydrofurfuryl acrylates cited and used in EP 0 343 467 B1. A further efficient crosslinking concept is the copolymerization of UV photoinitiators into the polyacrylate chain. For example, benzoin acrylate has been used as a comonomer and the crosslinking has been conducted on the backing using UV light [DE 27 43 979 A1]. In U.S. Pat. No. 5,073,611, on the other hand, benzophenone and acetophenone were used as copolymerizable monomers.
An entirely new method has come from chip manufacture. By adding a photocationic initiator and carrying out irradiation with UV light, polymers containing ester functions are deprotected and free carboxylic acids or hydroxyl functions are generated. This method has been described in U.S. Pat. No. 4,491,628. More recent developments deal with the change in the UV light wavelength to 193 nm [U.S. Pat. No. 5,910,392] and with improving the resolution [U.S. Pat. Nos. 5,861,231; 4,968,581; 4,883,740; 4,810,613; 4,491,628].
It is an object of the invention to provide a polyacrylate whose intermediates in the course of processing feature a low viscosity and thus great ease of processing in the hotmelt process, and which is converted only after processing, e.g., in a hotmelt process, into a composition of high shear strength. Through an appropriate choice of the reaction parameters during the conversion reaction, the intention is that it should be possible to adjust the physical properties of the polymer. The reaction causing the conversion should be able to be conducted when the polymer composition is already present in its ultimate form: for example, applied as an adhesive composition on a backing.
By the introduction of surface structures into the acrylic PSA compositions (introduction of polar alongside nonpolar regions) during UV irradiation, the intention is to obtain an additional improvement in the adhesive properties of the acrylic PSA compositions, a particular intention being to improve the bond strength and the tack in respect of the uniform polyacrylate mixtures. The aim of the invention is therefore to produce hard segments by the UV-initiated deprotection reaction of tert-butyl acrylate to give the free carboxylic acid by means of selective UV irradiation through a mask.
This object is achieved by means of a polyacrylate in accordance with the main claim. The subclaims relate to advantageous developments of the invention and to a process for preparing the polyacrylate, and also to its use.
The invention accordingly provides structured polyacrylates preparable by crosslinking, induced by UV irradiation, of a polymer mixture comprising the following components:
a) polyacrylate copolymers of the following monomers
a1) acrylic acid and/or acrylic esters of the following formula
CH2=CH(R1)(COOR2),
where R1=H or CH3 and R2 is an alkyl chain having 1-20 carbon atoms, at 70-99% by weight, based on component (a),
a2) olefinically unsaturated monomers containing functional groups, at 0-15% by weight, based on component (a),
a3)tert-butyl acrylate, at 1-15% by weight, based on component (a), and
b) a photocationic initiator at 0.01-25% by weight, based on the overall polymer mixture, the structuring of the polyacrylates comprising the presence in the polyacrylates of regions of high crosslinking alongside regions of low crosslinking and/or noncrosslinked regions.
It is particularly advantageous if component (a1) is present at 85-89% by weight, based on component (a), and/or component (a2) is present at 4-6% by weight, based on component (a), and/or component (a3) is present at 7-9% by weight, based on component (a).
It is very advantageous to use a polymer mixture in which up to 3% by weight, based on the overall polymer mixture, of
c) a difunctional or polyfunctional crosslinker is added.
Crosslinkers which can be used here are all difunctional or polyfunctional compounds whose functional groups are able to enter into a linking reaction with the polyacrylates, especially addition polymerization, polycondensation or polyaddition reactions. Preferably, these reactions will take place on a carboxyl group. Particularly suitable crosslinkers are epoxides or isocyanates containing at least two functional groups, although all other compounds which react with carboxyl groups are also suitable. It is also possible to use metal chelate compounds for this purpose.
A process for preparing structured polyacrylates by structured crosslinking of polyacrylate mixtures comprises irradiating the base polymer mixture with ultraviolet light in such a way that only certain regions of the polymer mixture are exposed to the UV radiation.
The preparation process may in particular be conducted such that the base polymer mixture is irradiated with ultraviolet light through a perforated mask in such a way that only certain regions of the polymer mixture are exposed to the UV radiation.
Alternatively, the structuring of the polymer mixture for curing may be achieved by using, rather than the perforated mask, a film whose two-dimensional extent has regions of different UV light permeability, so that certain regions of the polymer mixture are exposed to different intensities of the UV radiation.
The invention further provides for the use of the polyacrylate as a pressure-sensitive adhesive composition, in particular its use as a pressure-sensitive adhesive composition for an adhesive tape, the acrylic pressure-sensitive adhesive composition being present as a single- or double-sided film on a backing sheet.
As backing materials for adhesive tapes, for example, it is possible in this context to use the materials customary and familiar to the skilled worker, such as sheets (polyester, PET, PE, PP, BOPP, PVC), nonwovens, wovens and woven sheets, and also release paper, if desired. This list is not intended to be conclusive.
The invention set out above makes it possible to achieve the stated object. A polymer mixture is presented which is present in low-viscosity form during processing and in which the shear strength can be increased after it has been applied, for example, to a backing, this increase in shear strength being achieved by converting nonpolar into polar groups. The flow viscosity of the starting polyacrylates is very low and is therefore highly suitable for the hotmelt process, for example. By selective conversion of the groups it is possible to generate polar segments alongside nonpolar segments, leading to an improvement in the adhesive properties of the product.
The principles of the invention are set out below. The starting polymers may be prepared by a free radical polymerization, and their molecular weight is of the order of 1 000 000. For the starting polymer mixture it is possible to use any polymer which has adhesive properties in accordance with the Handbook of Pressure-sensitive Adhesives, p. 172, xc2xa71, 1989. As an essential component for the deprotection reaction which is to take place subsequently, polymers prepared using tert-butyl acrylate as comonomer must be present, since the conversion of the nonpolar into polar groups is based on the replacement of the tert-butyl group by protons.
The chemical reaction whose consequence is the conversion of nonpolar into polar PSA compositions on the backing is a deprotection of the tert-butyl acrylate to give the free carboxylic acid, in the presence of protons, and has already been known for a long time [Loev, Chem. Ind. (London) 193 (1964)]. 
This reaction is used in the photoresistance technology (photoresists: light-sensitive, film-forming materials whose solubility behavior alters by exposure to light or other irradiation; in the case of negative working photoresists, this takes place by crosslinking or photopolymerization) for the deprotection of tert-butyl groups. In that technology, the acid is produced by the UV irradiation of an admixed photocationic initiator. In the context of this invention, this technology has been transferred to PSA compositions.
Cationic photoinitiators are typically aryldiazonium salts (xe2x80x9conium saltsxe2x80x9d), which may be represented in general by the formula Arxe2x88x92N=N+LXxe2x88x92, where LXxe2x88x92 is an adduct of a Lewis acid L and a Lewis base Xxe2x88x92. Particularly advantageous for LXxe2x88x92 are BF4xe2x88x92, SbF5xe2x88x92, AsF5xe2x88x92, PF5xe2x88x92 and SO3CF2xe2x88x92. Under the influence of UV radiation, there is rapid cleavage of the molecule into the aryl halide (ArX), nitrogen, and the corresponding Lewis acid. Diaryliodonium salts (C6H5)2I+LXxe2x88x92 and triarylsulfonium salts (C6H5)3S+LXxe2x88x92 are also known for use as cationic photoinitiators; in the presence of proton donors, they form strong (Brxc3x6nstedt) acids, which are likewise highly suitable for the initiation of cationic polymerizations.
Sulfonium salts as cationic photoinitiators are also present, for example, in the form of the compounds H5C6xe2x80x94COxe2x80x94CH2xe2x80x94S+LXxe2x88x92 or H5C6xe2x80x94COxe2x80x94CH2xe2x80x94Pyr+LXxe2x88x92, in which Pyr is a nitrogen-containing heteroaromatic system (e.g., pyridine, pyrimidine).
For the crosslinking reaction depicted, the photocationic initiators familiar to the skilled worker are used, preferably one of the initiators from the group set out in the above paragraph.